Slow sleep with Chrome, nearly solved
I finally tracked down why my iMac is so slow to go to sleep. With Chrome running, it takes ~30sec from the time I press the power button to when the machine actually sleeps. If I quit Chrome, it takes about 1sec. It turns out to be a Chrome bug that was fixed about a month ago and will appear in the next release (v31).
The bug is that a few of the Chrome child processes explicitly ask to for notifications from the Mac power manager, but fail to listen for and acknowledge those notifications. This caused the OS to wait until a timeout before sleeping. The fix was to delete those child processes from the list of those asking for power notifications. Doh!